In micrometer-sized magnetic thin films, the magnetization typically adopts an in-plane, circular configuration known as a magnetic vortex. At the vortex core, the magnetization turns sharply out of the plane, pointing either up or down. Magnetic data storage based on this binary phenomenon is an intriguing concept, but it would require the ability to flip the vortex cores on demand. Because these structures are highly stable, very strong magnetic fields of around half a tesla (approximately one-third the field of the strongest permanent magnet) were previously thought to be necessary to accomplish this. At the ALS, a team of researchers from Germany, Belgium, and the U.S. has used time-resolved scanning transmission x-ray microscopy (STXM) to observe vortex motion and demonstrate the feasibility of using weak magnetic fields as low as 1.5 millitesla (mT) to reverse the direction of a vortex core. The observed switching mechanism, which can be understood within the framework of micromagnetic theory, gives insights into basic magnetization dynamics and their possible application to data storage technologies.

The Superparamagnetic Limit and Beyond

The consumer electronics industry seems to have an endless supply of cool "must-have" products coming off the assembly line: Razrs, iPods, TiVos, Blu-rays, and of course, Macs and PCs. Each new gizmo must be smaller, or faster, or have quadzuple the capacity of the old model in order to sell. Is there a physical limit to how far this process can go? At the heart of each digital device is a way to physically store data in binary form (0/1, up/down, on/off). For magnetic media, binary digits (bits) have historically taken the form of grains of magnetic material in which all the spins are aligned. As we increase the number of bits squeezed into a finite area (areal density), a point is reached at which thermal vibrations are no longer negligible and can knock the bits out of alignment. This is called the superparamagnetic limit. While advances in technology have forestalled this limit for now (areal densities of up to 400 gigabits per square inch have been achieved in a laboratory setting), scientists are looking farther down the road at completely new ways to encode binary information. Magnetic vortex cores, being thermally stable and measuring just 10 nm across, offer one tantalizing possibility. Not only are these structures useful for applications such as data storage, they also form an interesting playground for fundamental studies of magnetism on a microscopic level.

Representation of a magnetic vortex.

In magnetic thin films, magnetostatic interactions usually force the magnetization to lie parallel to the film plane. When further constrained to an area of about a square micrometer or less, the magnetic moments will form rotationally symmetric patterns that follow closed flux lines. At the center, the tightly wound magnetization cannot lie flat, because the short-range exchange interaction favors a parallel alignment of neighboring magnetic moments. The direction of the out-of-plane component is defined as the polarization of the vortex core. Moreover, the vortex structure can be set into gyrotropic motion by the application of a small magnetic field, and the sense of the gyration (clockwise or counterclockwise) is determined by the vortex core polarization. Thus, a change in the sense of gyration unambiguously indicates a change in the vortex core polarization.

In this experiment, the researchers applied a small (0.1-mT) sinusoidal magnetic field to induce gyrotropic motion in a square Permalloy (Ni80Fe20) sample. The excitation frequency was set at 250 MHz, close to the resonance frequency of the system (about 244 MHz) derived from micromagnetic simulations. The excitation field was synchronized with flashes of circularly polarized x rays from ALS Beamline 11.0.2 to produce dynamic STXM images, with x-ray magnetic circular dichroism (XMCD) providing the contrast mechanism. A short (4-ns) burst of 1.5 mT was applied, superimposed on the weak alternating field. The results show that the vortex core follows an elliptical trajectory, and a change in the sense of the gyration (and thus the vortex core polarization) can be clearly seen after the burst.

Schematic of the sample setup. An alternating current Isin generates an in-plane magnetic field Hsin. Circularly polarized x-ray photons are selectively absorbed by the magnetic domains of the sample. An example of the magnetic contrast is shown at the right. The yellow arrows indicate the magnetization in the four domains.

Points on the sinusoidal magnetic field (green curve) correspond to x-ray flashes that record individual frames of the vortex core movie. When the frames are strung together, they reveal the sense of gyration of the vortex core. Before the burst, the gyration is clockwise, corresponding to a vortex core polarization pointing down. After the burst, the gyration is reversed, and the vortex core polarization points up.

This vortex core switching, observed experimentally for the first time, was also reproduced by micromagnetic simulations. The simulations show that the burst distorts the out-of-plane vortex structure and creates a region of opposite out-of-plane magnetization at the edge of the original vortex core. This opposite magnetization grows and eventually splits into a vortex–antivortex pair with equal polarizations. The newly formed vortex and antivortex move apart, and the antivortex moves toward the original vortex. When the antivortex meets the original vortex, they annihilate each other, emitting spin waves in the process, until only one vortex, having a reversed polarization, remains in the structure.

Micromagnetic simulation of the reversal of the vortex core excited with a short magnetic field pulse. In the initial state, the vortex core is pointing down (a). Under influence of the external field, a region forms with opposite magnetization (b) and reaches full amplitude (c) before a double peak is created: a vortex–antivortex pair (d). The antivortex moves toward the original vortex (e) and annihilation occurs, creating spin waves (f–h). A vortex core with opposite polarization remains (i).

These results show that properly tuned bursts of only 4 ns can be used to switch the polarization of a vortex core. Because the resonance frequency of the gyrotropic mode scales inversely with the lateral dimensions, much shorter pulses should be sufficient for switching the core polarization of smaller elements. Although their practical realization is still far off, data storage systems based on this core-switching scheme could have several advantages, including high thermal stability, insensitivity to external static fields, and minimal crosstalk between neighboring patterns, all of which are indispensable features for ultrahigh-density magnetic storage devices.